A level restoration circuit in a trans-impedance amplifier removes the DC component, i.e. the average value which carries no information, of an optical signal exiting an optical fiber onto a photodiode in a receiver optical sub-assembly (ROSA), while at the same time keeping the low frequency −3 dB frequency low enough to meet requirements for both Telecom and Data com applications.
With reference to FIG. 1, a conventional TIA circuit, generally indicated at 1, converts the current IPD exiting a photodiode 2, into an output voltage VOUT. The photodiode current IPD, which enters the TIA circuit 1 at an input terminal 3, includes both a DC component and an AC component. The AC component, which carries the information, must be maintained and sent down an amplification chain 4 to final receiving equipment (not shown), while the DC component should be reduced and if possible eliminated, since many front end unit input circuits are not designed to tolerate much more than 10 μA without additional circuitry. A feedback circuit, generally indicated at 5, removes the DC component by means of negative feedback, e.g. implemented by a feedback amplifier 6/low pass filter (i.e. Capacitor 7) and a bypass transistor 8 combination. There are other ways of removing the DC component of the signal and the present invention will work with many methods of feedback DC removal. The feedback amplifier 6/low pass filter 7 has gain, and removes the AC component of a voltage feedback signal VFB, leaving only a DC component VFBDC. The capacitor 7 is used to set the low-frequency cutoff that the TIA circuit I requires. The low-frequency cutoff point prevents desired output signal components being subtracted from the input signal. The bypass transistor 8 takes that DC component VFBDC of the voltage feedback signal VFB and generates a DC current IFBDC in the collector 9, which by the action of negative feedback substantially equals the incoming DC current IPDDC from the photodiode 2. Accordingly, the DC component IPDDC is removed from the incoming signal IPD and passed to the ground GRND through the emitter 11 of the bypass transistor 8.
Unfortunately, the photodiode DC current IPD can vary from about 10 uA up to about 1000 uA, which makes a 40 dB of difference between the low and high values. To maintain good performance, the DC component must be subtracted with sufficient accuracy over this entire range. For this reason, the gain of the feedback circuit 5 must be made large enough to account for this wide range of input current. In general, if the DC component of IPD could be maintained at a known, constant value, the feedback circuit 5 would only need to account for the offset generated by the on-chip circuit components. Adding the 40 dB range of the input current, IPD, requires an additional 40 dB of gain in the feedback circuit 5. In general, for single-pole systems, increasing the gain in the feedback loop by 20 dB requires that the filtering capacitor 7 be multiplied by a factor of 10 to maintain the same low-frequency response. Accordingly, the 40 dB range of IPD requires 100 times more capacitance making it 100 times more difficult and expensive to integrate on-chip.
U.S. Pat. Nos. 6,404,281, issued Jun. 11, 2002 in the name of Kobayashi et al; U.S. Pat. No. 6,504,429, issued Jan. 7, 2003 to Kobayashi et al; and U.S. Pat. No. 6,771,132 issued Aug. 3, 2004 to Denoyer et al disclose improvements to TIA feedback circuits that include minimizing the upper limit of the low frequency cut off frequency; however, none of these references uses the photodiode power monitor circuit to cancel the dc offset current.
An object of the present invention is to overcome the shortcomings of the prior art by providing a circuit that cancels the dc offset current from a photodiode outside of the offset cancellation feedback loop enabling the TIA's offset cancellation circuitry to be subjected to a drastically narrower input current range and therefore requiring a relatively small gain over the same range of external photodiode input currents.